In parallel with the discussion on the \(L_{X} - L_{opt}\)  relation, a debate on the mass-to-light (M/L) ratio of FGs arose. In fact, if there is a chance that FGs are under luminous in the optical bands, they should have larger M/L ratios than non-FGs [1]}. Again, the studies available in the literature are somewhat contradictory. [2]}, [3]}, [4]} found normal values for their samples of FGs, compatible with non-fossil systems, although if marginally darker for a fixed mass. On the other hand, [5]} found a high M/L for their sample of overluminous elliptical galaxies (OLEGs, about 3 times larger). The same result was found by [6]} in analysing a sample of 10 FG candidates. The authors suggested that FGs are simply dark clusters: they are characterised by a mass and a central galaxy that are typical of galaxy clusters, but embedded in a poor environment, so that the richness and the total optical luminosity are below the non-fossil ones. Finally, [7]} also found high M/L ratios for their sample of “isolated X-ray overluminous elliptical galaxies”.
However, we suggest that inconsistencies in the measured M/L ratios could be due to differences in the methodology and quality of the data. For example, the optical luminosities of [5]} and [4]} are computed in the R band and within \(r_{200}\)  for every object in the sample, those of [6]} in a variable radius of \(500-1000\)  kpc, whereas [2]} estimated a normal M/L ratio from the gas fraction profile out to 450 kpc, and [7]} uses the B-band luminosity of the BCG as the total luminosity of the system.
Thus, it seems reasonable that the disagreement between different results could be healed only with a homogeneous study of a large sample of FGs. As we already mentioned for other topics, such a study is far from being performed.
